Overboost shutdown @ 13 psi?

[Subliminal]

Hey guys. I finally got my boost gauge hooked back up, after spending a week trying (and eventually suceeding) to stop overboost shutdown.

Well, today, I backed off my diode to let the map function normally, to see where overboost shutdown happens. It seems like it's happening @ around 13 psi. Isn't this a little bit low?

It is possible that the gauge isn't right, but it registers 0 at 0 and -18 at idle. That would make a guy assume that it's pretty durn close.

Any ideas on that one?

Thanks!

damon

[glhsken]

Depending on the computer, that is where the shutdown occurs.

[Subliminal]

Really? I thought it was always 14.7. This is on an 89 2.5 t1.

Oh well, doesn't really matter, i just thought it was a little weird.

The big can, 20s regulator and rail are going on this weekend, so we'll see what we can do about that crappy 13 psi shutdown.

[Leafy]

Interesting, because I have an 89 2.5 T1 and I seem to hit boost cut at 14.7.

[glhsken]

boost cut out eliminator

Good stuff there

[Subliminal]

Ken,

That particular thread seems to be more about the variance in zener diodes, rather than a variance in map sensors.

If my boost gauge is correct, the computer is cutting fuel when it sees 13 psi, rather than the full 14.7.

[glhsken]

There is a post there explaining this... it references gary's page...

[Subliminal]

There's two posts there that reference Gary's page.

The first one basically says that a 4.3v won't give you all your fuel, but a 4.7 might be too much and still hit overboost. I would assume this is because of the loose tolerances of the $.50 zener diodes.

The second one basically says the same thing.

I'm still a little confused. Could ya spell it out for the slow one (me)?

[glhsken]

this is the key statement

"If you have a stock computer, you must use the 4.3v diode. If you use the 4.7v the boost reading the computer still sees is higher than the programmed boost "target" and you will still get overboost shutdown. You must have a MP computer to use the 4.7v diode, they have a higher "target" boost voltage and won't shut down at the 4.7v map sensor reading"

basically, mp computers shut down at 14.7psi based on the tables. Non mp or stock computers have a lower programmed "shutdown" point. Not sure exactly what thap psi is, but it would be lower than 14.7.

[Subliminal]

Roger that...read between the lines, damnit Damon!

I wonder if they intentially did that so that their performance computers actually held a performance edge?

[Shel-Game]

On the stock 2.5 T1 ('89 MSEC and '90/91 SBEC), OB shutdown is 13.6 psi.

The MP computers for the 2.5 T1 have the OB set at 14.0psi.

[glhsken]

Thanks!!!

[turbovanman]

Quote:

Originally Posted by Shel-Game On the stock 2.5 T1 ('89 MSEC and '90/91 SBEC), OB shutdown is 13.6 psi. The MP computers for the 2.5 T1 have the OB set at 14.0psi.

Weird! for my mini, it was around 14.7 psi??????

[Shel-Game]

Quote:

Originally Posted by turbovanman Weird! for my mini, it was around 14.7 psi??????

As read on the guage? I think they are only accurate to 0.5psi or so.

Also, the computer calculates boost above baro, but the guage is (usually) calibrated to standard atmosphere (1-bar, 14.7psi). So, if your local barometric pressure is higher than 1-bar (say 14.9psi), then the guage would read higher than 14.0 psi, even though the computer would only be calculating 14.0 psi boost.

[turbovanman]

Quote:

Originally Posted by Shel-Game As read on the guage? I think they are only accurate to 0.5psi or so. Also, the computer calculates boost above baro, but the guage is (usually) calibrated to standard atmosphere (1-bar, 14.7psi). So, if your local barometric pressure is higher than 1-bar (say 14.9psi), then the guage would read higher than 14.0 psi, even though the computer would only be calculating 14.0 psi boost.

Sea level, autometer boost guage. Close enough to 15 psi so I figure 14.7 was accurate!

But as people are finding out, every car/van is different!